1. Field of the Invention
The present invention relates to a displacement encoder.
2. Description of Related Art
Currently, as a type of an apparatus for measuring a displacement, an optical displacement encoder is known. The optical displacement encoder includes a scale and a detection head that moves along the scale. The scale is provided with, for example, an absolute pattern for detecting a reference position and an incremental pattern for detecting a relative displacement between the scale and the detection head. The optical displacement encoder determines the reference position by using a reference signal which is a result of the detection of the absolute pattern on the scale. Further, a positional relation between the scale and the detection head can be detected by taking account of (i.e., can be detected based on) a displacement relative to the reference position obtained from a detection result of the incremental pattern.
In general, the incremental pattern is formed as a diffraction grating in which a plurality of grating patterns are arranged in a measurement direction. Light is applied (i.e., emitted) to this diffraction grating and an optical intensity of interference fringes that are formed by interference between +1st order diffracted light and −1st order diffracted light diffracted by the diffraction grating is detected.
In this type of optical displacement encoder, in order to accurately detect interference fringes between the +1st order and −1st order diffracted light, it is necessary to prevent or minimize the effect on the interference fringes caused by diffracted light having the other orders such as 0th order diffracted light.
For example, a displacement encoder in which 0th order diffracted light is removed by disposing an optical block between a light source and a scale has been proposed (Japanese Patent No. 2619566). In this displacement encoder, an index grating is interposed between the light source and the scale and light is applied from the light source to the index grating. A shield for blocking the 0th order diffracted light is interposed between the index grating and the scale. This shield is disposed in such a position that the 0th order diffracted light is blocked but the +1st order and −1st order diffracted light are not blocked. Therefore, while the +1st order and −1st order diffracted light reach the scale, the 0th order diffracted light does not reach the scale. As a result, only the +1st order and −1st order diffracted light travel from the scale to a detection unit, thus making it possible to prevent the effect of the 0th order diffracted light.
Further, another example of a displacement encoder using an index grating has been proposed (Japanese Patent No. 4856844). In this displacement encoder, light is applied from a light source to a scale and diffracted light that has passed through the scale is detected. An index grating is interposed between the scale and a detection unit. Further, a diffraction grating is formed (i.e., disposed) only in a place on which +1st order and −1st order diffracted light of the diffracted light coming from the scale are incident, so that the other orders of diffracted light including 0th order diffracted light are blocked. The +1st order and −1st order diffracted light incident on the index grating are diffracted by the diffraction grating and interference fringes are formed on the detection unit. In this way, only the +1st order and −1st order diffracted light travel from the scale to the detection unit, thus making it possible to prevent the effect of the 0th order diffracted light.
Further, a displacement encoder in which 0th order diffracted light is removed by using a spatial filter has been proposed (Kazuhiro Hane, others: 2, “Optical Encoder Using Metallic Surface Grating”, Journal of the Japan Society for Precision Engineering, Vol. 64, No. 10, 1998). In this displacement encoder, laser light is applied to a scale and resultant diffracted light is collimated by a collimator lens. Then, a spatial filter in which a slit is disposed in such a place that only +1st order and −1st order diffracted light of the collimated diffracted light having various orders coming from the collimator lens pass therethrough is used, so that the other orders of diffracted light including 0th order diffracted light are blocked. After that, by converging the +1st order and −1st order diffracted light on a detection unit by a convergence lens, interference fringes can be formed on the detection unit.